1. Field of the Invention
The present invention relates to a novel immersion tin/lead alloy plating bath and, more specifically, to an immersion tin/lead alloy plating bath preferably used for plating copper and copper alloys.
2. Prior Art
Immersion tin/lead alloy plating can be applied to the copper of independent circuit patterns in which it is difficult to make the portions to be plated electrically conductive, such as electronic parts and geometrically complicated parts or printed circuits.
Conventional immersion tin/lead alloy plating bath compositions known so far in the art contain stannous chloride and lead chloride as metal salts, thiourea as a chelating agent and hydrazine hydrochloride as a reducing agent (Japanese Patent Laid-Open Publication No. 64527/1974) or tin borofluoride and lead borofluoride as metal salts, thiourea as a chelating agent, sodium hypophosphite as a reducing agent and a surface active agent, borofluoric acid, etc. as additives (Japanese Patent Laid-Open Publications Nos. 211565/1984 and 211566/1984). Another bath is available, which contains an organic sulfonic acid and its divalent tin and lead salts, sodium hypophosphite as a reducing agent and thiourea as a chelating agent together with hydroxycarboxylic acids and various surface active agents (Japanese Patent Laid-Open Publication No. 184279/1989).
As mentioned above, various compositions have so far been developed as immersion tin/lead alloy plating baths, but they are generally so slow in the deposition rate of tin/lead that an ample deposit thickness cannot be obtained; that is, the obtained deposit thickness is of the order of 1 .mu.m. In addition, this type of tin/lead alloy plating may be achieved at various tin/lead ratios, as is well known in the art. Although it is now desired to obtain a deposit having an Sn/Pb ratio of 60%/40% or so, because it has the lowest melting point and so can be easily soldered at low temperatures, with the conventional plating baths it is difficult to provide stably deposits especially at this Sn/Pb ratio. Further, the temperatures of the plating baths lie in a relatively high temperature region, say, ordinarily 60.degree. to 80.degree. C.
An object of the present invention is therefore to solve the problems associated with the prior art by providing an immersion tin/lead alloy plating bath which can produce a tin/lead alloy film of enough thickness and improved adhesion and homogeneity onto copper or a copper alloy at various Sn/Pb ratios, especially an Sn/Pb ratio of 60%/40% or so in a relatively low temperature region within a short period of time.